Re: [PATCH 17/17] xfs: support for synchronous DAX faults

On Fri 27-10-17 08:16:11, Dave Chinner wrote: > On Thu, Oct 26, 2017 at 05:48:04PM +0200, Jan Kara wrote: > > > > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c > > > > index f179bdf1644d..b43be199fbdf 100644 > > > > --- a/fs/xfs/xfs_iomap.c > > > > +++ b/fs/xfs/xfs_iomap.c > > > > @@ -33,6 +33,7 @@ > > > > #include "xfs_error.h" > > > > #include "xfs_trans.h" > > > > #include "xfs_trans_space.h" > > > > +#include "xfs_inode_item.h" > > > > #include "xfs_iomap.h" > > > > #include "xfs_trace.h" > > > > #include "xfs_icache.h" > > > > @@ -1086,6 +1087,10 @@ xfs_file_iomap_begin( > > > > trace_xfs_iomap_found(ip, offset, length, 0, &imap); > > > > } > > > > > > > > + if ((flags & IOMAP_WRITE) && xfs_ipincount(ip) && > > > > + (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP)) > > > > + iomap->flags |= IOMAP_F_DIRTY; > > > > > > This is the very definition of an inode that is "fdatasync dirty". > > > > > > Hmmmm, shouldn't this also be set for read faults, too? > > > > No, read faults don't need to set IOMAP_F_DIRTY since user cannot write any > > data to the page which he'd then like to be persistent. The only reason why > > I thought it could be useful for a while was that it would be nice to make > > MAP_SYNC mapping provide the guarantee that data you see now is the data > > you'll see after a crash > > Isn't that the entire point of MAP_SYNC? i.e. That when we return > from a page fault, the app knows that the data and it's underlying > extent is on persistent storage? > > > but we cannot provide that guarantee for RO > > mapping anyway if someone else has the page mapped as well. So I just > > decided not to return IOMAP_F_DIRTY for read faults. > > If there are multiple MAP_SYNC mappings to the inode, I would have > expected that they all sync all of the data/metadata on every page > fault, regardless of who dirtied the inode. An RO mapping doesn't Well, they all do sync regardless of who dirtied the inode on every *write* fault. > mean the data/metadata on the inode can't change, it just means it > can't change through that mapping. Running fsync() to guarantee the > persistence of that data/metadata doesn't actually changing any > data.... > > IOWs, if read faults don't guarantee the mapped range has stable > extents on a MAP_SYNC mapping, then I think MAP_SYNC is broken > because it's not giving consistent guarantees to userspace. Yes, it > works fine when only one MAP_SYNC mapping is modifying the inode, > but the moment we have concurrent operations on the inode that > aren't MAP_SYNC or O_SYNC this goes out the window.... MAP_SYNC as I've implemented it provides guarantees only for data the process has actually written. I agree with that and it was a conscious decision. In my opinion that covers most usecases, provides reasonably simple semantics (i.e., if you write data through MAP_SYNC mapping, you can persist it just using CPU instructions), and reasonable performance. Now you seem to suggest the semantics should be: "Data you have read from or written to a MAP_SYNC mapping can be persisted using CPU instructions." And from implementation POV we can do that rather easily (just rip out the IOMAP_WRITE checks). But I'm unsure whether this additional guarantee would be useful enough to justify the slowdown of read faults? I was not able to come up with a good usecase and so I've decided for current semantics. What do other people think?